Project Summary The genetic basis of complex traits, especially behavioral traits, is often difficult to dissect because the effects of genes often depend on the genetic background in which they occur. Additionally, genes are typically involved in multiple phenotypes (they are pleiotropic) making the context of where/when they are expressed important to understanding their impacts on phenotype. Speciation offers an opportunity to study complex traits because when new lineages diverge there is strong selection for female behavior so that females reject males of the ?wrong? lineage. This strong selection results in genetic changes that can be identified and then the impacts of these changes can be dissected. One such case occurs in ?races? of Drosophila melanogaster. There is strong reproductive isolation between populations that are from the ancestral range (Southern Africa) and cosmopolitan populations. Females from the ancestral range reject cosmopolitan males. One candidate gene for this behavior, desat2, has been identified. Alleles of African D. melanogaster are pleiotropic; they confer female preference and contribute to differences in female pheromones (cuticular hydrocarbons). It remains unknown how much variation in female preference is controlled by desat2, and which mechanisms affect female behavior. I will study how desat2 influences female mating behavior by first examining how genetic background interacts with different desat2 alleles to affect female behavior. I will engineer strains that all share a common desat2 allele but differ in their genetic background. This will allow me to determine how genetic background alters the impact of desat2 on female behavior (Aim1). I will then look at the effects of desat2 on female specific pheromones (cuticular hydrocarbons) and expression patterns of desat2. To look at the role of female cuticular hydrocarbons I will engineer flies that express desat2 but do not produce female cuticular hydrocarbons. I can then add precise amounts of cuticular hydrocarbons back to the flies and ask how they affect behavior (Aim2). Lastly, I will assay tissue specific expression of desat2 in neuronal tissue using quantitative PCR and high-resolution microscopy. Expression in specific neuronal tissue will suggest that desat2 contributes to female perception of CHCs (Aim3). The outcomes of this research will provide insight into how pleiotropic traits and genetic background contribute to complex behavioral phenotypes.